Functionality of modern smart munitions such as destroyer war ships including DDG 1000, high energy lasers, counter MANPAD systems, and laser guided missiles, depend on the availability of special window or dome materials that are transparent in the visible through infrared (IR) wavelength region of the electro-magnetic spectrum. To be effective, these materials must combine IR transmission property with high strength and durability as well as being amenable to cost effective large scale manufacturing and precision fabrication into small and large components to exacting specifications.
Sapphire has found wide spread use in such modern military applications as electro-optic sensor windows and reconnaissance windows. Its strength and durability enable it to meet the demanding requirements of military applications, such as rain and sand erosion, heat, vibration, g-loading and shock resistance, and, in naval applications, resistance to waveslap loads. Sapphire is a single crystal grown from molten aluminum oxide in the shape of a cylindrical boule at high temperature, from which windows and domes have to be fabricated by slicing, cutting, grinding and polishing. Sapphire, has a combination of good mechanical properties and mid IR transparency in the 0.2 to 4.5 micron wavelength range, but also suffers from certain limitations. Monocrystalline sapphire is difficult and expensive to grow, particularly in large sizes. Further, with specific regard to surveillance aircraft window applications, it is undesirably heavy and suffers from optical birefringence due to its anisotropic rhombohedral crystal structure. Moreover, domes, lenses and other three-dimensional shapes must be cored out from boules, which makes it extremely expensive and almost unaffordable in many of the applications.
Therefore, a need exists to provide an alternative material to sapphire that is highly transparent and durable, while being easier to manufacture, of lighter weight, and avoiding the limiting optical birefringence issues associated with sapphire's anisotropic crystal structure.